1. Field of the Invention
The present invention relates to the extraction of acidic aqueous phases containing sulfate ions, in particular sulfuric acid, and additionally titanium, iron and in particular iron (II) ions, and possibly other metallic impurities. More especially, this invention relates to the extraction of solutions of residual sulfuric acid, which notably result from the sulfate process for the production of titanium dioxide, to respectively recover the sulfate/titanium and iron values therefrom.
2. Description of the Prior Art
It is of course a truism that sulfuric acid is one of the most widely used chemicals in industry. In many cases, the acid is dumped after use, which gives rise to pollution problems which become increasingly severe as the amounts of acid used increase.
Certain industrial processes and in particular the sulfate process for the production of pigmentary titanium dioxide produce large amounts of aqueous sulfuric acid, in a typical concentration by weight of from 10% to 50%, containing metal sulfates such as ferrous sulfates and titanium, as impurities. Such acid solution may also contain substantial amounts of other metal salts.
Said metal salts are in fact formed as by-products, for example, in the production of TiO.sub.2, when the titaniferous raw material such as ilmenite is subjected to attack by, or acidulation with sulfuric acid, such metal salts being salts of aluminum, vanadium, chromium and manganese, in particular.
The iron content, primarily in the ferrous state, is generally in the range of from 5 to 100 g/l, while the titanium content is from 1 to 20 g/l.
It will be appreciated that the presence of one or more of the aforementioned metal salts in the residual aqueous acid gives rise to the danger of causing problems in regard to dumping it into a river or other land waters, or into the sea.
It may therefore be necessary to recover such residual acids, and such a recovery operation is moreover advantageous since, particularly as regards the production of titanium dioxide, such acids may represent 33% of the acid employed for acidulation of the ore.
However, the technique of concentrating a residual acid such as to permit its reuse for acidulating ilmenite, for example, 65% H.sub.2 SO.sub.4, is seriously hindered by virtue of the precipitation of impurities, which causes the evaporator units to foul.
Moreover, it has also been found that it is an attractive prospect to recover the titanium contained in the residual acid. In fact, when producing titanium dioxide, the residual waters may contain a not inconsiderable portion of the production. For example, for a facility producing 180 t/day of TiO.sub.2, between 5 and 25 t/day of TiO.sub.2 contained in the residual waters is discharged into the sea.
A number of processes for treating acid solutions containing metal impurities by extraction by means of organic solvents such as neutral phosphoric acid esters and amines is known to the art.
However, it to date has proven impossible to develop a process enabling both purification of the residual solution and the recovery of titanium values therefrom.